Macro with_nested 

macro_rules! with_nested {
    ($field:expr, $type:ty, $var:ident => $body:block) => { ... };
}

Macro to handle optional nested config initialization in builders.

This macro eliminates the repeated pattern of:

// if self.config.field.is_none() {
//     self.config.field = Some(Type::new());
// }

Usage

// Instead of:
// if self.config.orientation.is_none() {
//     self.config.orientation = Some(DocOrientationClassifierConfig::new());
// }
// if let Some(ref mut config) = self.config.orientation {
//     config.confidence_threshold = Some(threshold);
// }

// Use:
// with_nested!(self.config.orientation, DocOrientationClassifierConfig, config => {
//     config.confidence_threshold = Some(threshold);
// });